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Chapter 5: Problem 31

What is meant by an entbalpy change?

Short Answer

Expert verified
Answer: Enthalpy change (鈭咹) represents the difference in enthalpy between the final and initial states of a system, usually caused by a chemical reaction or a phase change. It helps us understand heat transfer during chemical reactions and phase changes. The different types of enthalpy change include enthalpy of reaction, enthalpy of formation, enthalpy of combustion, enthalpy of fusion, enthalpy of vaporization, enthalpy of solution, and enthalpy of neutralization. Studying these enthalpy changes allows chemists to predict the behavior of systems and design better industrial and chemical applications.

Step by step solution

01

Definition of Enthalpy

Enthalpy (H) is a measure of the total energy of a thermodynamic system. It is expressed in units of joules (J). Enthalpy includes internal energy, pressure, and system volume, and is an extensive property, meaning it depends on the quantity of matter in the system. It is useful for understanding heat transfer during chemical reactions and phase changes.
02

Enthalpy Change

Enthalpy change (鈭咹) represents the difference in enthalpy between the final and initial states of a system, usually caused by a chemical reaction or a phase change. Positive enthalpy change indicates that the system has absorbed energy (endothermic process), while negative enthalpy change indicates that the system has released energy (exothermic process).
03

Types of Enthalpy Change

Enthalpy change can be categorized into different types, depending on the process occurring: 1. Enthalpy of reaction (鈭咹r): The enthalpy change during a chemical reaction taking place at constant pressure. 2. Enthalpy of formation (鈭咹f): The enthalpy change when one mole of a compound is formed from its elements in their standard states. 3. Enthalpy of combustion (鈭咹c): The enthalpy change when one mole of a substance is completely burned in oxygen at constant pressure. 4. Enthalpy of fusion (鈭咹fus): The enthalpy change when one mole of a solid is converted to its liquid state at its melting point. 5. Enthalpy of vaporization (鈭咹vap): The enthalpy change when one mole of a liquid is converted to its vapor state at its boiling point. 6. Enthalpy of solution (鈭咹soln): The enthalpy change when one mole of solute is dissolved in a solvent to form an infinitely dilute solution. 7. Enthalpy of neutralization (鈭咹neut): The enthalpy change when an acid and a base react to form one mole of water. These different types of enthalpy change help chemists understand and predict the behavior of systems during chemical reactions and phase changes. By studying enthalpy changes, we can also determine the efficiency and sustainability of certain processes and design better industrial and chemical applications.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Thermodynamic System
In the realm of physics, a thermodynamic system is essentially a confined space in which energy transactions occur. This can come in the form of heat or work and encompasses a spectrum of matter, which could be a gas, a liquid, a solid, or a mixture of phases.

One can visualize a thermodynamic system as a sealed container that is able to engage with its surroundings, known as the environment, through various forms of energy exchange. Importantly, it is categorized into three types:
  • Closed systems, which exchange energy (in the form of heat or work) but not matter with their surroundings.
  • Open systems, which can exchange both energy and matter.
  • Isolated systems, which do not exchange either energy or matter with the environment.
Understanding the type of system is crucial when we consider enthalpy change, as the conditions under which reactions take place dictate how energy is transferred and thus how enthalpy is calculated.
Endothermic and Exothermic Processes
Energy flow in chemical reactions is central to the concepts of endothermic and exothermic processes. An endothermic process is characterized by the absorption of heat, resulting in an increase of the system's enthalpy. Imagine ice melting on a warm day; energy is absorbed from the environment to change the state of the ice, thus being endothermic.

On the flip side, an exothermic process releases heat, causing a decrease in enthalpy. A common example of this is combustion, like a candle burning, where the released energy can be felt as heat. In the context of the enthalpy change, the sign convention plays a vital role: a positive change in enthalpy (*) signals an endothermic reaction, while a negative change (symbol) signals an exothermic reaction.
Types of Enthalpy Changes
Understanding the various types of enthalpy changes is fundamental to predicting and controlling chemical processes. Let's explore a few key types:
  • The Enthalpy of Reaction (_r): This is the energy change during a chemical reaction at constant pressure and is indicative of the reaction's energetic demands or releases.
  • The Enthalpy of Formation (_f): It represents the energy required to form one mole of a compound from its elements in their standard states, shedding light on the relative stability of different compounds.
  • The Enthalpy of Combustion (_c): It refers to the energy change when a substance combusts completely in oxygen, important for energy production analysis.
  • The Enthalpy of Fusion (_fus) and Vaporization (_vap): These are related to phase changes, the former for melting and the latter for boiling.
  • The Enthalpy of Solution and Neutralization: These are crucial for understanding dissolving processes and acid-base reactions, respectively.
Each type plays an integral role in fields like chemistry and engineering, helping to drive innovations in material science, energy resources, and beyond.

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Most popular questions from this chapter

Calculate \(\Delta E\) for a. the combustion of a gas that releases \(210.0 \mathrm{kJ}\) of heat to its surroundings and does \(65.5 \mathrm{kJ}\) of work on its surroundings. b. a chemical reaction that produces \(90.7 \mathrm{kJ}\) of heat but does no work on its surroundings.

Burning \(1.43 \mathrm{g}\) of methane \(\left[\mathrm{CH}_{4}(g), \text { molar mass } 16.0 \mathrm{g} /\right.\) mol] with excess \(\mathrm{O}_{2}\) raised the temperature of \(250 \mathrm{g}\) of water in a bomb calorimeter from \(22^{\circ} \mathrm{C}\) to \(98^{\circ} \mathrm{C}\). Calculate the molar enthalpy of combustion of methane.

The destruction of the ozone layer by chlorofluorocarbons (CFCs) can be described by the following reactions: $$\begin{aligned} \mathrm{ClO}(g)+\mathrm{O}_{3}(g) \rightarrow \mathrm{Cl}(g)+2 \mathrm{O}_{2}(g) & & \Delta H_{\mathrm{rxn}}=-29.90 \mathrm{kJ} \\ 2 \mathrm{O}_{3}(g) \rightarrow 3 \mathrm{O}_{2}(g) & & \Delta H_{\mathrm{rxn}}=24.18 \mathrm{kJ} \end{aligned}$$ Determine the value of the heat of reaction for the following: $$\mathrm{Cl}(g)+\mathrm{O}_{3}(g) \rightarrow \mathrm{ClO}(g)+\mathrm{O}_{2}(g) \quad \Delta H_{\mathrm{rxn}}=?$$

For which of the following reactions does \(\Delta H_{\mathrm{rxn}}^{\circ}\) also represent an enthalpy of formation? a. \(2 \mathrm{N}_{2}(g)+3 \mathrm{O}_{2}(g) \rightarrow 2 \mathrm{NO}_{2}(g)+2 \mathrm{NO}(g)\) b. \(\mathrm{N}_{2}(g)+\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{NO}(g)\) c. \(2 \mathrm{NO}_{2}(g) \rightarrow \mathrm{N}_{2} \mathrm{O}_{4}(g)\) d. \(\mathrm{N}_{2}(g)+2 \mathrm{O}_{2}(g) \rightarrow 2 \mathrm{NO}_{2}(g)\)

Use the following data to determine whether the conversion of diamond into graphite is exothermic or endothermic: $$\begin{array}{rll} \mathrm{C}(s, \text { diamond })+\mathrm{O}_{2}(g) \rightarrow \mathrm{CO}_{2}(g) & \Delta H^{\prime} & =-395.4 \mathrm{kJ} \\ 2 \mathrm{CO}_{2}(g) \rightarrow 2 \mathrm{CO}(g)+\mathrm{O}_{2}(g) & \Delta H^{\prime} & =566.0 \mathrm{kJ} \\ 2 \mathrm{CO}(g) \rightarrow \mathrm{C}(s, \text { graphite })+\mathrm{CO}_{2}(g) & \Delta H^{\circ} & =-172.5 \mathrm{kJ} \\ \mathrm{C}(s, \text { diamond }) \rightarrow \mathrm{C}(s, \text { graphite }) & \Delta H^{\circ} & =? \end{array}$$
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